Soil Ecosystem Processes Research Articles

Biodiversity Effects on Soil Processes Explained by Interspecific Functional Dissimilarity

The loss of biodiversity can have significant impacts on ecosystem functioning, but the mechanisms involved lack empirical confirmation. Using soil microcosms, we show experimentally that functional dissimilarity among detritivorous species, not species number, drives community compositional effects on leaf litter mass loss and soil respiration, two key soil ecosystem processes. These experiments confirm theoretical predictions that biodiversity effects on ecosystem functioning can be predicted by the degree of functional differences among species.

Within-trophic group interactions of bacterivorous nematode species and their effects on the bacterial community and nitrogen mineralization

Knowledge of the interactions between organisms within trophic groups is important for an understanding of the role of biodiversity in ecosystem functioning. We hypothesised that interactions between bacterivorous nematodes of different life history strategies would affect nematode population development, bacterial community composition and activity, resulting in increased N mineralization. A microcosm experiment was conducted using three nematode species (Bursilla monhystera, Acrobeloides nanus and Plectus parvus). All the nematode species interacted with each other, but the nature and effects of these interactions depended on the specific species combination. The interaction between B. monhystera and A. nanus was asymmetrically competitive (0,-), whereas that between B. monhystera and P. parvus, and also A. nanus and P. parvus was contramensal (+, -). The interaction that affected microcosm properties the most was the interaction between B. monhystera and P. parvus. This interaction affected the bacterial community composition, increased the bacterial biomass and increased soil N mineralization. B. monhystera and P. parvus have the most different life history strategies, whereas A. nanus has a life history strategy intermediate to those of B. monhystera and P. parvus. We suggest that the difference in life history strategies between species of the same trophic group is of importance for their communal effect on soil ecosystem processes. Our results support the idiosyncrasy hypothesis on the role of biodiversity in ecosystem functioning.

Soil microbial communities under conventional-till and no-till continuous cotton systems

Soil management practices affect soil microbial communities, which in turn influence soil ecosystem processes. In this study, the effects of conventional- (fall disking, chiseling and spring disking, field cultivation) and no-tillage practices on soil microbial communities were examined under long-term continuous cotton ( Gossypium hirsutum L.) systems on a Decatur silt loam soil. Soil samples were taken in February, May, and October of 2000 at depths of 0–3, 3–6, 6–12, and 12–24 cm. Compared to the conventional-till treatment, the no-till treatment increased soil organic carbon and total nitrogen contents in the surface layer by 130 and 70%, respectively. Microbial biomass C content under no-till treatment was 60, 140, and 75% greater than under conventional-till treatment in February, May, and October, respectively. Principal components analysis of phospholipid ester-linked fatty acid (PLFA) profile indicated soil microbial communities shifted over time and with soil depth. This change appeared to be driven primarily by soil bacterial populations as indicated by the major PLFA contributors (i.e. fatty acids 16:0, 10Me16:0, cy19:0, 16:1 2OH, and i15:0) to the first two principal components. Tillage treatment differences were revealed by analysis of variance on the first principal components (PC 1), which accounted for 62% of the total sample variance, and by the relative abundance of selected PLFAs and PLFA ratios. The impact of tillage practices was significant in February and May, but not in October. During the growing season, changes in the microbial community may be primarily determined by soil conditions responding to cotton growth and environmental variables such as moisture and temperature; during fallow or prior to cotton establishment, community changes associated with tillage practices become more pronounced. These findings have implications for understanding how conservation tillage practices improve soil quality and sustainability in a cotton cropping system.

C and N mineralisation in the decomposer food webs of a European forest transect

Belowground processes are essential for the overall carbon and nitrogen fluxes in forests. Neither the functioning of the soil food web mediating these fluxes, nor its modulation by environmental factors is sufficiently understood. In this study the belowground carbon and nitrogen mineralisation of four European coniferous forest sites (northern Sweden to north‐east France) with different climate and N depositional inputs was analysed by investigating the soil food webs using field observations and modelling. The soil fauna directly contributed 7–13% to C mineralisation, among which the testate amoebae (Protozoa) made the largest contribution. Microbial grazing was suggested to have an important indirect effect by stimulating bacterial turnover. Due to relatively high C:N ratios of their substrate, bacteria immobilized N, while the fauna i.e. testate amoebae, nematodes, microarthropods and enchytraeids, counteracted this N immobilisation.Despite similar food web biomass, the sites differed with respect to food web structure and C and N flows. Model calculations suggested a significant influence of food web structure on soil ecosystem processes in addition to environmental factors and resource quality. Mineralisation rates were lowest at the low N input boreal site with a food web dominated by fungal pathways. Further south, as N availability increased, bacterial pathways became more important and the cycling of C and N was faster. The bioavailability of degradable C sources is suggested to be a limiting factor for microbial activity and overall mineralisation rates. In this respect, above‐ and belowground interactions e.g. transfers of labile C sources from the vegetation to the decomposer system deserve further attention.Our study revealed the combined effects of climate and nutrient inputs to ecosystems and the subsequent changes in the structure and functioning of the systems. If decomposition, and therefore carbon loss, is stimulated as a consequence of structural and/or nutritional changes, resulting for example from continuous industrial N emission, the storage capacity of forest ecosystems could be altered.

The use of integrated soil microcosms to predict effects of pesticides on soil ecosystems

Most studies that aim at assessing the effects of pesticides on soil organisms or soil ecosystem processes are related to a single species of organism or soil process. Such individual studies are usually performed according to standard test guidelines, prepared by national or international authorities or approved test organizations and used in risk assessment. Over the last four years, with scientists from Germany and Russia, we have tested integrated soil microcosms, as model terrestrial ecosystems, to assess simultaneously the overall effects of a single pesticide, on a range of representative soil organisms, ecosystem processes, and environmental fate. This integrated approach takes account of interactions between organisms and processes which may influence the overall environmental impact and fate of a pesticide. Results from a detailed study of the environmental impact of the fungicide carbendazim are presented and some results on the impact of copper on soil systems are also reviewed. Some ecosystem structural parameters that were affected include: microbial activity, nematode communities, earthworm numbers and masses and plant growth. Some of the ecosystem processes affected include: nitrogen mineralization, and nutrient transformations.

Modeling soil thermal and carbon dynamics of a fire chronosequence in interior Alaska

In this study, the dynamics of soil thermal, hydrologic, and ecosystem processes were coupled to project how the carbon budgets of boreal forests will respond to changes in atmospheric CO2, climate, and fire disturbance. The ability of the model to simulate gross primary production and ecosystem respiration was verified for a mature black spruce ecosystem in Canada, the age‐dependent pattern of the simulated vegetation carbon was verified with inventory data on aboveground growth of Alaskan black spruce forests, and the model was applied to a postfire chronosequence in interior Alaska. The comparison between the simulated soil temperature and field‐based estimates during the growing season (May to September) of 1997 revealed that the model was able to accurately simulate monthly temperatures at 10 cm (R> 0.93) for control and burned stands of the fire chronosequence. Similarly, the simulated and field‐based estimates of soil respiration for control and burned stands were correlated (R= 0.84 and 0.74 for control and burned stands, respectively). The simulated and observed decadal to century‐scale dynamics of soil temperature and carbon dynamics, which are represented by mean monthly values of these variables during the growing season, were correlated among stands (R= 0.93 and 0.71 for soil temperature at 20‐ and 10‐cm depths,R= 0.95 and 0.91 for soil respiration and soil carbon, respectively). Sensitivity analyses indicate that along with differences in fire and climate history a number of other factors influence the response of carbon dynamics to fire disturbance. These factors include nitrogen fixation, the growth of moss, changes in the depth of the organic layer, soil drainage, and fire severity.

Influence of decomposer food web structure and nitrogen availability on plant growth

We studied the sensitivity of soil microbial communities and ecosystem processes to variation in the vertical and horizontal structure of decomposer food web under nitrogen poor and N-enriched conditions. Microcosms with humus and litter layer of boreal forest floor, birch seedlings infected with mycorrhizal fungi, and decomposer food webs with differing trophic group and species composition of soil fauna were constructed. During the second growing period for the birch, we irrigated half of the microcosms with urea solution, and the other half with de-ionised water to create two levels of N concentration in the substrate. During the experiment night time respirations of the microcosms were measured, and the water leached through the microcosms was analysed for concentration of mineral N, and nematode numbers. The microcosms were destructively sampled after 37 weeks for plant biomass and N uptake, structure of soil animal and microbial community (indicated by PLFA profiles), and physical and chemical properties of the humus and litter materials. Predatory mites and nematodes had a negative influence on the biomass of their microbivorous and microbi-detritivorous prey, and microbi-detritivores affected the biomass and community structure of microbes (indicated by PLFA-analysis). Moreover, predatory mites and nematodes increased microbial biomass and changed the microbial community structure. The decomposer food web structure affected also N uptake and growth of plants. Microbi-detritivorous fauna had a positive effect, whereas predators of microbial and detritus feeding fauna exerted a negative influence on plant N uptake and biomass production. The impact of a trophic group on the microbes and plant was also strongly dependent on species composition within the group. Nitrogen addition magnified the influence of food web structure on microbial biomass and plant N uptake. We suggest that addition of urea-N to the soil modified the animal-microbe interaction by increasing microbial growth and altering community structure of microbes. The presence of microbi-detritivores and predators reduced loss of carbon from the microcosms, and the food web structure influenced also water holding capacity of the materials. The changes in plant growth, nutrient cycling, size of N and C pools, and in the physical properties of the soil emphasize the importance and diversity of indirect consequences of decomposer food web structure.

Nematode communities as indicators of status and processes of a soil ecosystem influenced by agricultural management practices

Nematode communities were monitored for three years in a citrus soil ecosystem in Central Florida under various agricultural regimes comparing standard vs. reduced-input practices. Differences in agricultural regimes consisted of two fertilization levels, two irrigation levels, and two types of ground cover under the tree (herbicide vs. mulch). While some nematodes were affected sporadically by fertilization and irrigation treatments, mulch had a consistent and frequently significant effect on many bacterivores, fungivores, herbivores, and omnivores. Rhabitidae, Cephalobus, Aphelenchus, and Aphelenchoides had an immediate but temporary response to mulch additions. Acrobeles, Acrobeloides, Eucephalobus, Teratocephalus, Criconemoides, Aporcelaimellus, and Eudorylaimus were always less abundant in mulch-treated plots, whereas Plectus and Belonolaimus were always more abundant. Of various indices of community composition, only maturity indices, unlike diversity indices, indicated the status and intensity of soil processes (decomposition, mineralization). However, different responses of single genera within a trophic group implied unique contributions of nematode genera in soil ecosystem processes on a temporal scale, suggesting that generic or possibly species level of resolution provide the most adequate information about the soil ecosystem.

Soil community composition and ecosystem processes Comparing agricultural ecosystems with natural ecosystems

Soil organisms play principal roles in several ecosystem functions, i.e. promoting plant productivity, enhancing water relations, regulating nutrient mineralisation, permitting decomposition, and acting as an environmental buffer. Agricultural soils would more closely resemble soils of natural ecosystems if management practices would reduce or eliminate culti- vation, heavy machinery, and general biocides; incorporate perennial crops and organic material; and synchronise nutrient release and water availability with plant demand. In order to achieve these goals, research must be completed to develop methods for successful application of organic materials and associated micro-organisms, synchronisation of management practices with crop and soil biota phenology, and improve our knowledge of the mechanisms linking species to ecosystem processes.

SPATIOTEMPORAL DISTRIBUTIONS OF BACTERIVOROUS NEMATODES AND SOIL RESOURCES IN A RESTORED RIPARIAN WETLAND

Spatial and temporal variability in soil biotic populations reflect heterogeneity in soil resources, affect patterns of soil process rates, and facilitate coexistence of diverse biota. We investigated these relationships in a 0.7-ha restored riparian wetland in the Coastal Plain of Georgia, USA, for an abundant and diverse group of soil fauna, the bacterivorous nematodes. We quantified spatial distributions in four different seasons for the eight most dominant bacterivorous taxa in the wetland and related their individual distributions to patterns of microbial respiration, inorganic nitrogen, moisture, and soil organic matter. We used geostatistics to quantify spatial aggregation and draw isopleths. For all variates except two nematode taxa, 36–99% of sample population variance was spatially dependent, over ranges of 11–84 m. Isopleths and spatial trend analysis showed that individual bacterivorous taxa exhibited divergent spatial distributions, with populations aggregating into different hotspots in the wetland. Although these large-scale trends persisted at all sampling dates, small-scale patterning showed significant temporal variation due to rise and fall of local populations. Individual nematode distributions did not correspond well to the (temporally more static) soil resource patterns, except occasionally to soil moisture and nitrate content. We attribute the general lack of correlation between nematode and soil resource patterns in part to the young age (2.5–3.5 yr) of the investigated wetland site. Although nematode patterns remain inadequately explained, we suggest that the observed spatiotemporal divergence among populations of bacterivorous taxa has important implications for our understanding of soil ecosystem and community processes, notably the spatiotemporal distribution of nematode-influenced nitrogen cycling rates and the maintenance of field-scale nematode diversity.

Soil fauna, guilds, functional groups and ecosystem processes

Soil ecology has much to contribute to our understanding of important processes at the ecosystem level such as primary production as affected by the rhizosphere biota, organic matter dynamics and nutrient cycling and soil structure dynamics. Soil animals play an important part in these processes, but it has always been difficult to single out their contributions. In studies of these processes soil ecologists often rely on `guilds' and `functional groups' as their basic unit of observation. Here, I evaluate the usefulness and limitations of this approach for understanding some of the above-mentioned processes. A simple conceptual model of ecological interactions in soil is presented. The plant has a central place in this model in that it both governs, and is a biological assessment of, the roles soil fauna play in ecosystem processes. The need for a quantitative model of ecological interactions in soil is discussed with reference to current modelling efforts of organic matter and food webs in soil.

Impacts of rising atmospheric carbon dioxide on model terrestrial ecosystems

In model terrestrial ecosystems maintained for three plant generations at elevated concentrations of atmospheric carbon dioxide, increases in photosynthetically fixed carbon were allocated below ground, raising concentrations of dissolved organic carbon in soil. These effects were then transmitted up the decomposer food chain. Soil microbial biomass was unaffected, but the composition of soil fungal species changed, with increases in rates of cellulose decomposition. There were also changes in the abundance and species composition of Collembola, fungal-feeding arthropods. These results have implications for long-term feedback processes in soil ecosystems that are subject to rising global atmospheric carbon dioxide concentrations.

Regulation of decomposer community structure and decomposition processes in herbicide stressed humus soil

Regulation of soil decomposer community structure and ecosystem processes, such as nutrient cycling, under herbicide stress was studied in a microcosm experiment. For the experiment, coniferous forest soil was defaunated and put into the microcosms. In the microcosms two different food webs including microbes, nematodes, tardigrades and oribatid mites, either with or without predatory mesostigmatid mites, were reconstructed. Half of the microcosms were stressed with a herbicide (active ingredient was terbuthylazine). During the 57 weeks incubation community structure of decomposers and nitrogen mineralisation were studied at five destructive samplings and two water irrigations. Soil respiration was measured weekly starting at week 26. Mesostigmatid mites regulated densities of some prey species and hence they had an effect on the community structure of microbivores. A trophic cascade from predators to microbes took place both in unstressed and stressed soils: microbial activity decreased in the presence of predators. Predation effect was observed more clearly in the unstressed soil although predators maintained their populations longer in the herbicide stressed soil. Predators had no significant effects on N mineralisation while herbicide increased it. Oribatids were reduced by the herbicide at the later phase of the experiment. It can be concluded that decomposer food webs and decomposition in the soil can partly be top-down controlled. Due to a high degree of omnivory it was impossible to determine precisely the trophic structure of the food web. Herbicide contamination altered the community regulation and ecosystem processes via direct toxicity and by affecting trophic interactions. Although the application of food web analysis in risk assessment procedures has been proved to be problematic, there is a clear need for system level studies because of the chemical-induced indirect effects on the food webs.

Soil Food Webs and Ecosystem Processes: Decomposition in Donor-Control and Lotka-Volterra Systems

We analyze how populations and food web structure affect the decomposition process. Based on a theory of carbon dynamics at the ecosystem level and food web theory at the population level, a link between population- and ecosystem-level theories is generated through a series of factors connecting population parameters to ecosystem-level decomposition rates. These factors are the ratio of the steady state biomass of a functional group to the mass of its resource, the total mortality rate of a functional group, and a carbon recycling index defined for each living component in the decomposer system. The factors defined at the primary decomposer level embody information about the whole food web. Two different assumptions about the mechanisms of trophic interactions in the food web are used: donor-control and Lotka-Volterra. The results show how the structure of the soil community and the characteristics and properties of soil fauna contribute to the decomposition process. Under the assumption of donor-controll...

Sustained Productivity of Forests Is a Continuing Challenge to Soil Science

AbstractScientific uncertainties about ecosystem processes and greater awareness of the need for environmental care are sources of public anxiety over forest management. Partly because of this, the negative impacts of poor forestry practices are often emphasized, overlooking the achievements in sustainable forestry. The forest estates should be viewed and managed as a continuum so that the overall need for production of wood and the protection of environmental values can be met. We need practical goals of forest management. One goal should be to ensure that the trend in forest productivity is nondeclining or is positive through successive rotations and harvests, while maintaining and enhancing the quality of the soil resource base in perpetuity. The conflicts about the use of native forests for wood harvesting, while maintaining all conservation values, can be lessened if the value of each forest is ranked within a scale ranging from wood production to conservation. The growing demand for wood and concerns for land care can be met in part by expanding plantation forestry. Questions concerning management strategies for sustainable forestry are global in scope, but the genesis and application of practices for achieving this are local and are based fundamentally on the soil. The expectation of developing soil‐based sustainability indicators can be realized only if the expectation is backed by substantial research linking changes in soil properties, ecosystem processes, and productivity at the landscape level. Challenges are many and include interdisciplinary approaches to research and forest management, application to ensure economic prosperity, and positive approaches to communication.

Relevance of micro- and mesocosm experiments for studying soil ecosystem processes

The reliability of previously reported microcosm and mesocosm studies was tested by comparing the mean values of microbial respiration and enzyme activities and of the availability of macronutrients (K, Ca, Mg, NO 3, NH 4, PO 4) from these studies with the same variables in the forest floor. The field data show the same trend as the microcosm and mesocosm measurements, viz. a buffering effect of soil arthropods on forest floor processes, and a stimulated nitrification resulting in a lowered ammonium and calcium, and a higher nitrate availability. The use of microcosms and mesocosms is discussed. It is concluded that they are valuable tools to unravel the intrinsic soil ecosystem interactions, but mesocosms in the field are preferred to laboratory microcosms.

Respirometry System for Small Biological Samples

Soil organisms alter soil physical, chemical and biological properties in different ways. The composition and structure of biotic communities at one hierarchical level can influence the spatial heterogeneity of resource at other hierarchical levels, and the latter is supported by a number of biologically relevant spheres-functional domains in soil. In this paper, the role of soil biota in soil ecosystem processes was assessed, and the relationship between soil biodiversity and ecosystem function was presented. Soil ecosystem responses to global change were also discussed.